Meh, last time I checked, there were a lot of free demos out there (Unreal Tournament 2004 and Quake III Arena being two that I have downloaded, but I know a hell of a lot more exist), and people definitely still play them. I'm sometimes on as DeusXIII in both games, so if you see me, feel free to say hi.

Also, I don't know if you can play online, but I know there's a Halo demo out there. However, it didn't work on my system*, so check the specs.

*AthlonXP 2000+, cheap-ass integrated graphics card (S3 SavageProDDR or something), 480MB RAM (32MB shared w/video)

EDIT: Oh! Oh! I have something else to contribute!

A friend just told me about FlashFlashRevolution, which is a Flash version of DDR. It's spicy!